Various food products must be cooked until the internal temperature of the food reaches a predetermined temperature. This is especially true with respect to meat due to health purposes and personal preference. By achieving a certain internal temperature, the preparer can be reasonably certain that the food contains no living organisms which are known to have an immediate effect in many consumers. Also, the preparer can serve a dish that conforms to the personal preference of the consumer, i.e. rare, medium, or well-done without the need of cutting open the food to check or attempt to rely solely on time cooked.
Conventional food thermometers incorporate a sensing probe for insertion into the food. Thus, if a thermometer only indicates temperature, the food preparer must diligently check the thermometer to determine when the food has reached the desired temperature.
There exist various cooking thermometer type devices that provide an indicator, such as a pop-up element or color change, which indicate when a threshold temperature has been reached in accordance with a predetermined measurement. However, these devices are generally not adjustable or precisely accurate. Thus, the preparer must diligently check the indicator to determine when the indicator is activated.
Conventional thermometer devices used for cooking require the preparer to carefully and diligently check the device to prevent the food from becoming overcooked. Should the food preparer become distracted with other dishes, a phone call, etc. and forget to check the thermometer, reliance on conventional cooking thermometer devices may result in the food becoming overcooked and possibly inedible.
Existing mechanical meat thermometers typically use a bi-metal helical spring in a skewer together with a spur gear assembly to turn a temperature indicating pointer. When heated, the bi-metal helical spring expands and the resulting rotational motion moves a pointer attached to the end of the spring. Bi-metal helical springs of suitable size to be used in a meat thermometer provide only a minuscule amount of force, typically only to turn the pointer, but not enough to trigger a ringing mechanism.
To provide an improvement over prior art meat thermometers, the present invention provides a cooking thermometer having an audible alarm and a probe which incorporates a shape memory alloy segment of wire, such as Nickel Titanium (NiTi, commonly known as nitinol), instead of the bi-metal spring used in existing thermometers. Shape memory alloys undergo a thermoelastic phase transformation in their crystal structure when cooled from the stronger, high temperature form (Austenite) to the weaker, low temperature form (Martensite). This inherent phase transformation is the basis for the unique properties of shape memory alloys, including the shape memory effect, superelasticity, and high damping capability.
When a shape memory alloy is in its martensitic form it is easily deformed to a new shape. However, when the alloy is heated through its transformation temperatures, it reverts to austenite and recovers its previous shape with great force. The temperature at which the alloy remembers its high temperature form when heated can be adjusted by slight changes in alloy composition, mechanical working, and heat treatment.
The shape memory alloys also show a superelastic behavior if deformed at a temperature which is slightly above their transformation temperatures. This effect is caused by the stress-induced formation of some martensite above its normal temperature. Because it has been formed above its normal temperature, the martensite reverts immediately to undeformed austenite as soon as the stress is removed. This process provides a very springy “rubberlike” elasticity.
One of the properties of nitinol alloys is that they do not undergo their phase transformation at one particular temperature. Instead, the transformation begins at one temperature (known as the start temperature, MS,) and is completed at another temperature (known as the finish temperature, AF). Thus, the heat deformation properties of shape memory alloys can be utilized to operate a temperature measurement device. For optimum economy of design, it would be highly desirable to provide a cooking thermometer with an audible alarm in which a shape memory alloy is simultaneously used for both an analog temperature reading and also to provide the mechanical force necessary to trigger an alarm bell.